The present invention relates broadly to electromagnetic interference (EMI) shielding/grounding gaskets or seals, and particularly to a low closure force EMI shielding spacer gasket which is particularly adapted for use within small electronics enclosures such as cellular phone handsets and other handheld electronic devices.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed "electromagnetic interference" or "EMI" as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other "target" devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., "Gaskets That Block EMI," Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, silicone rubber, or fluorosilicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, plated glass, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression, transfer, or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
More recently, a form-in-place (FIP) process has been proposed for the manufacture of EMI shielding gaskets. As is described in commonly-assigned, co-pending application U.S. Ser. No. 08/377,412 filed Jan. 24, 1995, and in Canadian Patent Application 2,125,742, such process involves the application of a bead of a viscous, curable, electrically-conductive composition in a fluent state to a surface of a substrate such as a housing or other enclosure. The composition then is cured-in-place via the application of heat or with atmospheric moisture to form an electrically-conductive, elastomeric EMI shielding gasket in situ on the surface of the substrate. By forming and curing the gasket in place directly on the surface of the substrate, the need for separate forming and installation steps is obviated. Moreover, the gasket may be adhered directly to the surface of the substrate to further obviate the need for a separate adhesive component or other means of attachment of the gasket to the substrate. In contrast to more conventional die cutting or molding processes, the flashless FIP process reduces waste generation, and additionally is less labor intensive in that the need for hand assembly of complex gasket shapes or the mounting of the gasket into place is obviated. The process, which is marketed commercially under the name Cho-Form.RTM. by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass., further is amenable to an automated or roboticly-controlled operation, and may be employed to fabricate complex gasket geometries under atmospheric pressure and without the use of a mold.
Another recent EMI shielding solution for electronics enclosures is further described in commonly-assigned U.S. Pat. No. 5,566,055 and is marketed commercially under the name Cho-Shield.RTM. by the Parker Chomerics Division. Such solution involves the over-molding of the housing or cover with an conductive elastomer. The elastomer is integrally molded in a relatively thin layer across the inside surface of the housing or cover, and in a relatively thicker layer along the interface locations thereof providing both a gasket-like response for environmentally sealing the cover to the housing and electrical continuity for the EMI shielding of the enclosure. The elastomer additionally may be molded onto interior partitions of the cover or housing to provide electromagnetically-isolated compartments between potentially interfering circuitry components.
Yet another solution for shielding electronics enclosures, and particularly the smaller enclosures typical of cellular phone handsets and other handheld electronic devices, concerns the incorporation of a thin plastic retainer or frame as a supporting member of the gasket. The electrically conductive elastomer may be molded or, as is described in EP Patent No. 654,962, formed-in-place or otherwise attached to the inner or inner peripheral edge surfaces and/or to the upper or lower face surfaces of the frame. So constructed, the gasket and frame assembly may be integrated within the electronic device to provide a low impedance pathway between, for example, peripheral ground traces on a printed circuit board (PCB) of the device, and other components thereof such as the conductive coating of a plastic housing, another PCB, or a keypad assembly. Uses for the spacer gaskets of the type herein include EMI shielding applications within digital cellular, handyphone, and personal communications services (PCS) handsets, PC cards (PCMCIA cards), global positioning systems (GPS), radio receivers, and other handheld devices such as personal digital assistants (PDAs). Other uses include as replacements for metal EMI shielding "fences" on PCBs in wireless telecommunication devices. An example one of these applications is shown in FIG. 1 wherein a handset is shown at 10 as including a housing or other enclosure, 12, within which is received a pair of PCBs, 14a-b. A spacer gasket, referenced generally at 20, which includes a frame, 22, and an electrically-conductive, elastomeric member, 24, is inserted between PCBs 14 for providing, for example, a conductive pathway between one or more of the corresponding ground traces, one of which is designated at 26, of the boards.
Requirements for typical spacer applications generally specify a low impedance, low profile connection which is deflectable under relatively low closure force loads, e.g., about 1.0-8.0 lbs (2-16 kg) per inch of gasket length. Other requirements include low cost and a design which is adapted for automated assembly and which provides an EMI shielding effectiveness both for the proper operation of the device and for compliance, in the United States, with commercial Federal Communication Commission (FCC) EMC regulations. Spacer gaskets of the type herein involved are further described by Peng, S. H., and Tzeng, S. V., in "Recent Developments is Elastomeric EMI Shielding Gasket Design," and in the Parker Chomerics Technical Publication, "EMI Shielding and Grounding Spacer Gasket" (1996).
Heretofore, conventional EMI shielding spacer gasket cross-sections involved the generally flat or hemispherical cross-sectional profiles which are illustrated in cross-section in FIGS. 2A and 2B. In those figures, spacer gasket 20 of FIG. 1 reappears, respectively, as the prior art gaskets 20' and 20", with the prime and double prime designations being used throughout FIGS. 2A and 2B to distinguish the elements thereof, and with common reference to those elements being made in unprimed fashion. As is shown at 20 in FIGS. 2A and 2B, each of the conventional spacer gaskets includes a relatively thin retainer frame 22 which may be molded of a plastic material such as ABS, polycarbonate, nylon, polyester, polyetherimide, a liquid crystal polymer (LCP), or the like. A length of electrically-conductive elastomeric member 24, which usually is formulated as a silver or silver-plated-filled silicone or fluorosilicone, is bonded or otherwise retained along a peripheral edge or other surface, 26, of the frame 22. As in FIG. 2A, elastomeric member 24' may be injection or compression molded to the lands, 25a-b', of stepped frame edge surface 26'. Alternatively, as in FIG. 2B, elastomeric member 24" may be formed-in-place along the planar upper and lower face surfaces, designated at 26a" and 26b", of the frame 22".
When inserted between a pair of faying surfaces of a housing or circuit board assembly such as PCBs 14 of FIG. 1, the elastomeric member 24 of gasket 20 exhibits a characteristic deformation responsive to a closure or other deflection force transferred from the housing or PCB surfaces. Generally, a minimum deflection, typically of about 10%, is specified to ensure that the member sufficiently conforms to the mating housing or board surfaces to develop an electrically conductive pathway therebetween. It has been observed that for certain applications, however, that the closure or other deflection force required to effect the specified minimum deflection of the spacer gasket profiles heretofore known in the art is higher than can be accommodated by the particular housing or board assembly design. Thus, it will be appreciated that further improvements in the design of spacer gaskets profiles would be well-received by the electronics industry. As the sizes of handheld electronic devices such as cellular phone handsets has continued to shrink, especially desired therefore would be a low closure force gasket profile which is especially adapted for use in the smaller electronics enclosures which are rapidly becoming the industry standard.